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Categorization of Santa Ana Winds with Respect to Large Fire Potential

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Categorization of Santa Ana Winds with Respect to Large Fire Potential Categorization of Santa Ana Winds With Respect To Large Fire Potential Tom Rolinski1, Brian D’Agostino2, and Steve Vanderburg2 1US Forest Service, Riverside, California. 2San Diego Gas and Electric, San Diego, California. ABSTRACT Santa Ana winds, common to southern California during the fall through early spring, are a type of katabatic wind that originates from a direction generally ranging from 360°/0° to 100° and is usually accompanied by very low humidity. Since fuel conditions tend to be driest from late September through the middle of November, Santa Ana winds occurring during this time have the greatest potential to produce large, devastating fires when an ignition occurs. Such catastrophic fires occurred in 1993, 2003, 2007, and 2008. Because of the destructive nature of such fires, there has been a growing desire to categorize Santa Ana wind events in much the same way that tropical cyclones and tornadoes have been categorized. The Offshore Flow Severity Index (OFSI), previously developed by Predictive Services, is an attempt to categorize such events with respect to large fire potential, specifically the potential for new ignitions to reach or exceed 100 ha based on breakpoints of surface wind speed and humidity. More recently, Predictive Services has collaborated with meteorologists from the San Diego Gas and Electric utility to develop a new methodology that addresses flaws inherent in the initial index. Specific methods for improving spatial coverage and the effects of fuel moisture have been employed. High resolution reanalysis data from the Weather Research and Forecasting (WRF) model generated by the Department of Atmospheric and Oceanic Sciences at UCLA is being used to redefine the OFSI. In addition to the new methodology, social scientists from the Desert Research Institute have been contracted to evaluate how this index might best be conveyed to the user so as to maximize its effectiveness. This paper will outline the methodology for developing the improved index as well as discuss how it might benefit fire agencies, private industry, broadcast media groups and the general public. 1. Introduction or not a Santa Ana wind event occurred. This is a necessary process as it helps distinguish a true Santa From the fall through early spring, offshore winds, or Ana from the normal nocturnal offshore winds that what are commonly referred to as “Santa Ana” occur throughout the coastal and valley areas. winds, occur over southern California from the coastal mountains westward, from Ventura County During 21 through 23 October 2007, Santa Ana southward to the Mexican border. These synoptically winds generated multiple large catastrophic fires driven wind events vary in frequency, intensity, and across southern California (Moritz et al.2010). Most spatial coverage from month to month and from year notable was the Witch Creek fire in San Diego to year, thus making them difficult to categorize. County, where wind gusts of 26 m/s were observed at Most of these wind events are associated with mild to the Julian weather station along with relative warm ambient surface temperatures >= 18° C and humidity values of ≈ 5%. However, high resolution low surface relative humidity <= 20%. However, model simulations at 667 meters showed that wind during the late fall and winter months, these events velocities were much higher in unsampled areas tend to be associated with lower surface temperatures (Fovell 2012). This event generated an interest in due to the air mass over the Great Basin originating categorizing Santa Ana winds so that, with such an from higher latitudes. There are a variety of ways to index available, fire agencies and first responders, define a Santa Ana event through the analysis of local private industry, and the general public could be and synoptic scale surface pressure and thermal more informed about the degree of severity an event distributions across southern California (Raphael would have on the fire environment. This index could 2003). For our purposes, Mean Sea Level Pressure also help augment Fire Weather Watches and Red (MSLP) map types and surface wind speed Flag Warnings from the National Weather Service by observations will be the determining factors whether 1 providing value added information about an impending event. The Predictive Services Unit, functioning out of the Geographic Area Coordination Center in Riverside, California, is comprised of several meteorologists employed by the USDA Forest Service. In 2009, Predictive Services developed the Offshore Flow Severity Index (OFSI), which categorizes Santa Ana Figure 1 ‐ A 7‐day forecast of the Offshore Flow Severity wind events according to the potential for a large fire Index (OFSI) displaying categories of Santa Ana winds. to occur (Rolinski et al. 2011). This unique approach areas which have been divided into three zones (Fig. addresses the main impact Santa Ana winds could 1). Zone 1 covers the southern portion of Ventura have on the population of southern California beyond and Los Angeles Counties. Zone 2 consists of Orange experiencing the casual effects of windy, dry County, as well as western Riverside and western San weather. Bernardino Counties. Zone 3 represents most of San Diego County (Fig. 2). These zones were chosen in San Diego Gas and Electric (SDG&E) is a regulated part based on the different offshore flow utility provider across the southern portions of characteristics that occur across the region. For Orange County and all of San Diego County. During instance, Santa Ana winds across Zone 1 and Zone 2 the October 2007 event, it was determined that power are primarily a result of offshore surface pressure lines were the cause of several large fires, including gradients (locally and/or synoptically) interacting the Witch Creek fire in San Diego County. The cost with the local terrain to produce gap winds through of these fires to date currently exceeds 2 billion Soledad Canyon, the Cajon Pass, and the Banning dollars. As a result, SDG&E is investing in fire Pass (Hughes and Hall 2010; Fovell 2012). These weather related research and technology to develop winds also tend to precede the Santa Ana winds that an enhanced warning system for dangerous wildfire occur across San Diego County by 12 to 24 hours. conditions. Having advance notice of the severity Across Zone 3, offshore winds take on a more and timing of a Santa Ana wind event would permit “downslope windstorm” characteristic driven largely SDG&E to prepare, monitor, and deploy its resources by the tropospheric stability (Fovell 2012). In for maximum effectiveness. In order to accomplish addition, these zone boundaries were developed this, SDG&E is partnering with the local partially around political boundaries, as well as meteorological and fire community on this project. around the news media broadcast markets that cover the area. The OFSI has been proven to be successful in terms of capturing the overall nature of a Santa Ana wind event. However its basic method in addressing complex issues such as time, topography, and fuel conditions was perceived to be overly simplistic by the scientific community and also by other users of the index. These issues have since been addressed and will be discussed in detail in the following section. 2. Background A seven day forecast of the OFSI is currently being produced by Predictive Services on a daily basis for Figure 2 ‐ Map depicting OFSI Zones over southern the southern California coastal, valley, and mountain California. Favored wind corridors are indicated by yellow arrows. 2 Wind Velocity (mph) Wind Velocity (mph) 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 0 5 5 (%) 10 (%) 10 15 15 RH 20 RH 20 25 25 All Fires Fires >= 100 ha Figure 4 ‐ Scatter plot displaying days in which a fire Figure 3 ‐ Scatter plot displaying days in which a large (any size) occurred. fire (100 ha) occurred. 3. A New Methodology A large fire is defined as the 95th percentile of daily largest fires occurring over the past 20 years across a) Large Fire Potential – Meteorological the region depicted in figure 2. Thus a large fire for Conditions Zones 1, 2, and 3 is 100 ha, which is typically when The potential for an ignition to reach or exceed 100 additional resources will be required from outside the ha depends on a number of components: e.g. various point of origin to suppress the fire. This large fire meteorological and fuel conditions, suppression definition was used to develop the OFSI, which strategy, topography, accessibility, and resource correlates historical fire information with surface availability. Current methods to evaluate fire wind speed and humidity from Remote Automated potential include various indices from the National Weather Stations (RAWS), to initially form a four Fire Danger Rating System (NFDRS) and from the tier categorical index (Rolinski et al. 2011). Scatter Canadian Forest Fire Danger Rating System plots of wind velocity vs. relative humidity show the (CFFDRS) (Preisler et al. 2008). The Fosberg Fire frequency distribution of ignitions and 100 ha fires Weather Index (FFWI) is one such index which is a respectively over the dataset period during Santa Ana function of wind speed, humidity, and temperature wind events (Figures 3 and 4). Comparing these plots with output values ranging from 0 to 100 (Fosberg with each other revealed natural breakpoints in the 1978). While the FFWI may show elevated output data that were used to define the OFSI categories. values for a Santa Ana wind event, it can also show This process was repeated for multiple RAWS to elevated values for any day therefore making it too determine a site that would be most effective in generic for our purposes. The initial concept of OFSI representing each zone. Stations near the coast was a first attempt to create an index more specific to sometimes indicated offshore events as being weaker Santa Ana wind events, but further studies resulted in and of shorter duration than the mountains.
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